fix(codegen): Use wellknown types from prost-types (#49)

This change introduces the ability to let prost-build
provide the fully qualifed type path for types that
start with `google.protobuf`.

tonic-build/tests/wellknown.rs

@@ -0,0 +1,9 @@
+#[test]
+fn wellknown() {
+    let tmp = std::env::temp_dir();
+    tonic_build::configure()
+        .out_dir(tmp)
+        .format(false)
+        .compile(&["tests/protos/wellknown.proto"], &["tests/protos"])
+        .unwrap();
+}